1. Field of the Invention
The present invention relates to time slot interchange switches and, in particular, to a time slot interchange switch that includes a bit-error-rate test.
2. Background of the Invention
Time slot interchange switches are finding frequent use in telecommunications systems for switching of data from an input line to an output line. Typically, data is transmitted from a source to a destination over a channel. In a telephone communications network, for example, a channel can contain one voice conversation. At any moment in time, a voice channel can contain an n-bit (e.g., an 8-bit) representation of one sampling of the voice signals. In some embodiments, the analog voice signal can be sampled at a rate of about 8,000 times per second, although other sampling rates can be utilized.
Multiple channels can be placed on a single transmission line using Time Division Multiplexing (TDM). TDM places one channel from each input on the transmission line in a fixed sequence. A complete set of input samples, which includes any number of channels of data, is referred to as a frame. In one example of such a system, if the channel data is sampled at a rate of 8 kHz, the frame rate for the transmission line must also be 8 kHz. If a frame contains 32 channels, then the bit rate transmitted over the transmission line is 2.048 Mbps (8 kHz×32 channels×8 bits). Increasing the transmission line bit rate allows the number of channels in a frame to be increased.
FIGS. 1 and 2 illustrate the operation of a time slot interchange switch (TSIS) 100. As is illustrated in FIG. 1, TSIS 100 can be coupled to input data streams 110-1 through 110-4 and output data streams 120-1 through 120-4. As illustrated in FIG. 2, there can be any number N of input data streams 110 and output data streams 120. Each one of input data streams 110 can carry a channel in each time slot. For example, channel A0 of input stream 110-1 is in time slot 0. A channel is typically n-bits (for example 8-bits or 16 bits) and is typically transmitted serially.
As is illustrated in FIG. 1, TSIS 100 receives each of the channels in input data streams 110 and places the channels in a preprogrammed order in output data streams 120. For example, in the example shown in FIG. 1, channel A0 in time slot 0 of input data stream 110-1 is output in time slot 12 of output data stream 120-3. As is illustrated in FIG. 1, any channel carried in a time slot of input data streams 110 can be routed to any time slot of output data streams 120 by TSIS 100.
Therefore, TSIS 100 can be utilized to move channels in time as well as space. For example, input data streams 110 can be T1 lines, each of which typically carry 24 phone conversations. If some of the channels (which contain portions of individual conversations) need to be routed to a different T1 line to arrive at its appropriate destination, switching those channels to a separate T1 line can be accomplished without switching all of the conversations transmitted on the input T1 line. In some examples, further, an individual input stream 110 can be routed to multiple output streams 120 in a broadcast fashion.
FIG. 2 illustrates a system that can utilize TSIS 100. The input data streams 110-1 through 110-N can be, for example, T1 or E1 lines, sonnet STS3 lines, or any other transmission line that utilizies time domain multiplexing of channel data. As shown in FIG. 2, the input lines carry input data streams 110-1 through 110-N and output lines carry output data streams 120-1 through 120-N. Each of data streams 110-1 through 110-N can be received in an interface unit 130-1 through 130-N, respectively. Interface unit 130-1 through 130-N can be, for example, a T1 or E1 line interface unit (LIU), a SONET termination unit, or other interface to receive a data stream from a transmission medium. In general, interface unit 130-1 through 130-N filters and recovers transmission signals that are transmitted over the various transmission media. The data stream from each of interface units 130-1 through 130-N can then be received in a framer 131-1 through 131-N, respectively. Each of framers 131-1 through 131-N recognizes the incoming frame pulses and generates frame and clock information based on the received frame pulses. The frame and clock information from all of framers 131-1 through 131-N passes through multiplexer 132 to a phase-locked-loop (PLL) 133 that picks the best input to multiplexer 132 to use as a master clock. PLL 133 then generates timing signals, including a clock signal and a frame signal.
The master clock and frame pulse information is sent to all framers 132-1 through 132-N as well as to TSIS 100 and other system devices that deal with the incoming data stream. The data stream from framers 131-1 through 131-N is then input to TSIS 100. TSIS 100 then can switch channels received from input data streams 110-1 to 110-N to time slots amongst output data streams 120-1 through 120-N and couples the output data streams back to framers 131-1 through 131-N, respectively. Framer 131-1 through 131-N, then, adds framing pulses and provides output data streams 131-1 through 131-N to drivers 130-1 through 130-N for subsequent coupling to the output transmission medium.
As such, TSIS 100 can be utilized to switch T1/E1 voice or data channels in a conventional switching system. However, TSIS 100 can also be utilized in a modem switch bank to link incoming data from a local area network to modems. Further, TSIS 100 can be utilized in a wireless base station to connect cellular calls to the publically switched telephone network (PSTN). Further, multimedia gateways can utilize switches in networking environments involving multiple networks and data that includes voice, fax, video, or data.
Therefore, there is an ongoing need to provide time slot interchange switching with increasing capabilities.